Method for hot shaping a workpiece and agent for reducing the heat emission

ABSTRACT

A process for hot shaping a workpiece of metal or an intermetallic compound at a temperature of higher than about 1000° C. The method comprises at least partially coating the surface of the workpiece with a coating agent that comprises an oxide phase and an additive and/or an adhesive before processing the workpiece into a formed body or a rolling product. A coating agent for reducing the heat emission from the workpiece comprises a predominant amount of an oxide phase. This abstract is neither intended to define the invention disclosed in this specification nor intended to limit the scope of the invention in any way.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 of AustrianPatent Application No. A 878/2009, filed on Jun. 5, 2009, the disclosureof which is expressly incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for hot shaping throughsolid-blank forming such as forging or rolling a workpiece or rawmaterial of metal or of an intermetallic compound at a temperature ofmore than about 1000° C. The invention also relates to an agent for acoating to reduce the heat emission from a workpiece or raw materialheated to deforming temperature.

2. Discussion of Background Information

With materials with poor deformation properties a hot forming of aworkpiece of metal, such as an ingot or primarily formed raw material ofmetal or of intermetallic compounds to form a forging requires a precisetemperature control from heating up to the removal of the part from theforming means.

A sufficient workability of the material of the workpiece is often givenonly in a narrow temperature window, because lower forming temperatureslead to a brittleness and higher temperatures likewise lead to abrittleness and/or to a coarse grain formation of the microstructure ofthe workpiece.

As the case may be, the limit of a sufficient workability is at hightemperatures above 1000° C.

The emitted thermal energy increases in general with rising temperatureto the fourth power so that with high surface temperatures of theworkpiece the energy loss and the drop in temperature in the edge areain the unit of time are high.

With necessarily high forming temperatures it is therefore difficultand/or expensive to ensure a temperature with sufficient workability ofthe material also in the edge area of the workpiece over a necessaryperiod of time.

Workpieces are heated to forming temperature in the usual manner in afurnace. The heated part is subsequently removed from the furnace withknown means, conveyed to a shaping means, placed on a table roller or adie part and processed with dies in a forming manner. During this periodof time the surface of the workpiece emits heat and/or heat isdissipated into the dies.

The general problem therefore lies in a rapid loss of temperature of thezone of the workpiece close to the surface and an occurrence of defects,such as cracks, resulting therefrom.

To solve this problem it has already been proposed and is also practicedonce in a while to transfer the heated workpiece within a short periodof time. However, it is usually not possible to position the heatingunit and the forming device in immediate vicinity of each other.

Furthermore, an attempt has also already been made to heat the workpieceso much that even with a drop in temperature the surface zone thereof isstill in the temperature range of the workability of the material.However, in this manner a coarsening and/or deterioration of themicrostructure or center defects can occur.

Methods wherein the workpiece is enclosed in a capsule and is heated anddeformed therein are also known. A method of this type can be rewardingwith respect to a deformation of a part in a narrow temperature window,but requires a high expenditure.

In terms of process engineering, an isothermal forging of the workpieceis possible and useful in which the dies are heated to a temperatureclose to the forming temperature. However, a method of this type isextremely complex and expensive. It would be advantageous to haveavailable a method of the type mentioned at the outset for forming aworkpiece, which method overcomes the disadvantages of the knownmethods.

SUMMARY OF THE INVENTION

The present invention provides a process for hot shaping throughsolid-blank forming such as forging or rolling a workpiece or rawmaterial of metal or of an intermetallic compound at a temperature ofhigher than about 1000° C. The process comprises at least partiallycoating the surface of the workpiece or raw material with a coatingagent that comprises an oxide phase and an additive and/or an adhesive;allowing the coating thus applied to solidify; subsequently preheatingwith through-heating the coated workpiece or raw material to deformingtemperature and transferring same to a shaping device; and processing(e.g., forging and/or rolling) the workpiece or raw material with theshaping device into a formed body or a rolling product.

In one aspect of the process, the temperature of the surface of theworkpiece or raw material to be coated may be higher than about 100° C.For example, the temperature may be about 200° C.

In another aspect of the process, the workpiece or raw material may beimmersed into the coating agent and/or sprayed with the coating agent.

In yet another aspect, the solidified layer thickness of the coating maybe greater than about 0.1 mm, for example, from about 0.3 mm to about 3mm.

The present invention also provides a coating agent for reducing theheat emission from a workpiece or raw material of metal or anintermetallic compound which is heated to deforming temperature. Theagent comprises a predominant amount (e.g., more than about 50% byweight) of an oxide phase, one or more additives and one or more liquidcomponents.

In one aspect of the coating agent, the oxide phase thereof may comprisea metal oxide such as zirconium dioxide. For example, the agent maycomprise more than about 70% by weight, e.g., from about 80% to about98% by weight, or from about 90% to about 97% by weight, of zirconiumdioxide, based on the total weight of the coating agent.

In another aspect, the coating agent of the present invention maycomprise methylcellulose and/or microsilica as additive. For example,the agent may comprise (a) from about 0.1% to about 1% by weight, e.g.,from about 0.2% to about 0.7% by weight of methylcellulose and/or (b)from about 1% to about 10% by weight, e.g., from about 2% to about 8% byweight of microsilica, based on the total weight of the agent.

In yet another aspect, the coating agent of the present invention maycomprise sodium silicate glass as liquid component. For example, thecoating agent may comprise from about 15% to about 65% by weight, e.g.,from about 20% to about 60% by weight of sodium silicate glass, based onthe total weight of oxide phase and additive(s).

In a still further aspect of the coating agent of the present invention,the grain size of the oxide phase may be from about 1 μm to about 50 82m. For example, the mean grain size of the oxide phase may be about 12.5μm.

The present invention also provides a process for the hot forming ofparts of a gamma-titanium-aluminum base alloy by the process for the hotshaping of a workpiece according to the present invention as set forthabove (including the various aspects thereof) wherein a coating agentaccording to the present invention as set forth above (including thevarious aspects thereof) is employed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in the detailed descriptionwhich follows, in reference to the drawings by way of non-limitingexamples of exemplary embodiments of the present invention, and wherein:

FIG. 1 is graph showing the thermal expansion of a TiAl alloy and azirconium oxide coating as a function of the temperature;

FIG. 2 shows a specimen with the positions of the measuring points;

FIG. 3 is a graph showing cooling curves as a function of the time of acoated rod and a bare (uncoated) rod close to the surfaces thereof; and

FIG. 4 is a graph showing cooling curves as a function of the time of acoated rod and a bare rod of the rod cores.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The particulars shown herein are by way of example and for purposes ofillustrative discussion of the embodiments of the present invention onlyand are presented in the cause of providing what is believed to be themost useful and readily understood description of the principles andconceptual aspects of the present invention. In this regard, no attemptis made to show structural details of the present invention in moredetail than is necessary for the fundamental understanding of thepresent invention, the description taken with the drawings makingapparent to those skilled in the art how the several forms of thepresent invention may be embodied in practice.

The advantages achieved with the process according to the presentinvention include that in particular during the time of transfer of theworkpiece to the shaping device the radiation and thus the temperatureloss are reduced. This also applies to the placement of the workpieceonto a roller bed or onto a die part. It has been found, surprisingly toone skilled in the art, that a coated workpiece does not require longerheating times during heating in the furnace.

According to the present invention the coating of the workpiece isadvantageously carried out with a uniform layer thickness, withsubstantially no chipping of the layer occurring during heating andduring the subsequent transfer to the shaping means. The coating alsoreduces the heat transfer from the workpiece into the die at leastduring the first forming step.

A particularly good adhesion of the layer can be achieved according tothe invention if the coating of the surface of the workpiece or rawmaterial is carried out at a temperature of the same of more than about100° C., preferably at approximately 200° C.

If, as was found, a coating of the surface of the workpiece or rawmaterial is carried out by immersion into or spraying with a coatingagent, a largely uniform layer thickness can advantageously be achievedon the surface.

In order to achieve an optimum for a reduction of the radiation ofthermal energy from the surface and good layer adhesion on the one handand a desired high surface quality of the formed workpiece on the otherhand, it can be advantageous if the coating is carried out with asolidified layer thickness of greater than about 0.1 mm, preferably witha layer thickness of from about 0.3 mm to about 3 mm.

The agent for a coating for reducing the heat emission from a workpieceor raw material heated to deformation temperature, can be easily appliedin a thin layer and with a uniform layer thickness on the surface of aworkpiece before the heating, will not significantly flake off duringthe heating process in the furnace, has a sufficient adhesion during thetransfer to the die up to the first forming step and improves thequality of the forging.

According to the invention, the oxide phase of the coating agent acts asa heat-resistant insulating component, wherein one or more additives oradhesives which are present in smaller proportions bind the oxide grainsand hold them on the substrate. The one or more liquid components serveto homogenize the phases and adjust a desired degree of liquidity forthe homogenous application onto the surface of the workpiece or part.

An agent in which the main component or oxide phase is formed ofzirconium oxide with a proportion in % by weight of greater than about70, preferably from about 80 to about 98, in particular from about 90 toabout 97, has proven to be particularly advantageous with respect to amajor reduction of the heat emission.

Particularly if the proportion of zirconium dioxide is greater thanabout 70% by weight, an agent in which the additives methyl celluloseand/or microsilica are present in proportions in % by weight of fromabout 0.1 to 1, preferably from about 0.2 to about 0.7, and from about 1to about 10, preferably from about 2 to about 8, respectively, can beused particularly advantageously for a coating of TiAl alloys, becausethis type of alloy and the coating material have coefficients ofexpansion that differ only slightly from one another.

As a liquid component, sodium silicate glass with a proportion in % byweight of from about 15 to about 65, preferably from about 20 to about60 may be added to the agent, wherein this addition relates to the oxidephase with the additive(s).

An agent in which the oxide phase shows a grain diameter of from about 182 m to about 50 μm, preferably an average grain size of d₅₀=about 12.5μm, has proven to be particularly uniformly applicable, adhesive andeffective with respect to thermal insulation.

As mentioned above with respect to thermal expansion, the use of theabove-mentioned process for hot shaping a workpiece and the use of anagent according to the above specification for reducing heat emissionfrom a workpiece heated to deforming temperature for the hot forming ofparts of a gamma-titanium-aluminum-based alloy has proven to beparticularly advantageous, wherein a heating of this alloy to more thanabout 1280° C. can be undertaken and the substantially flawless coatingcauses a considerable reduction of the temperature drop of the zoneclose to the surface of the workpiece.

The invention is described in more detail based on results from thedevelopment work and from the comparative tests of the course of thetemperature over time on test pieces.

FIG. 1 shows the expansion of a substrate of a gamma-titanium-aluminumbased alloy and a zirconium coating as a function of the temperature upto 1000° C. Based on the figure it should be noted that the thermalexpansions of the two materials show only slight differences, which isthe reason for the substantial avoidance of a chipping of the layer fromthe base material.

FIG. 2 shows a specimen with a diameter of 40 mm Ø, which has a boreclose to the surface and a center bore for thermal elements.

The tests were carried out such that uncoated and coated specimen wereequipped with thermal elements and heated to a temperature ofapproximately 1290° C. After a through-heating the specimen were removedfrom the inert gas furnace, positioned on a fire-proof base, and thecourse of the temperature was measured as a function of the time.

FIG. 3 shows the temperature drop as a function of the time in the zoneclose to the surface of uncoated and coated specimen. Approx. 30 secondsafter the removal of the specimen an uncoated rod shows a temperature inthe surface area of approx. 1165° C. and a rod provided with a zirconiumdioxide layer shows a temperature of approx. 1215° C.

FIG. 4 shows the temperature drop in the centers of the specimen.

FIG. 3 and FIG. 4 do not require further explanation for one skilled inthe art and clearly show a heat emission reducing effect of a zirconiumoxide-based coating on a specimen of a gamma-titanium-aluminum-basedalloy.

It is noted that the foregoing examples have been provided merely forthe purpose of explanation and are in no way to be construed as limitingof the present invention. While the present invention has been describedwith reference to an exemplary embodiment, it is understood that thewords which have been used herein are words of description andillustration, rather than words of limitation. Changes may be made,within the purview of the appended claims, as presently stated and asamended, without departing from the scope and spirit of the presentinvention in its aspects. Although the present invention has beendescribed herein with reference to particular means, materials andembodiments, the present invention is not intended to be limited to theparticulars disclosed herein; rather, the present invention extends toall functionally equivalent structures, methods and uses, such as arewithin the scope of the appended claims.

1. A process for hot shaping through solid-blank forming a workpiece orraw material of metal or of an intermetallic compound at a temperatureof higher than about 1000° C., wherein the process comprises (a) atleast partially coating a surface of the workpiece or raw material witha coating agent that comprises an oxide phase and at least one of anadditive and an adhesive, (b) allowing the coating thus applied tosolidify, (c) subsequently preheating with through-heating the coatedworkpiece or raw material to deforming temperature and transferring sameto a shaping device, and (d) processing the workpiece or raw materialwith the shaping device into a formed body or a rolling product.
 2. Theprocess of claim 1, wherein the workpiece is at least one of forged androlled.
 3. The process of claim 1, wherein a temperature of the surfaceof the workpiece or raw material to be coated is higher than about 100°C.
 4. The process of claim 3, wherein the temperature is about 200° C.5. The process of claim 1, wherein in (i) the workpiece or raw materialis at least one of immersed into the coating agent and sprayed with thecoating agent.
 6. The process of claim 1, wherein a solidified layerthickness of the coating is greater than about 0.1 mm.
 7. The process ofclaim 6, wherein the thickness is from about 0.3 mm to about 3 mm.
 8. Acoating agent for reducing the heat emission from a workpiece or rawmaterial of metal or of an intermetallic compound which is heated todeforming temperature, wherein the agent comprises a predominant amountof an oxide phase, one or more additives and one or more liquidcomponents.
 9. The coating agent of claim 8, wherein the oxide phasecomprises a metal oxide.
 10. The coating agent of claim 9, wherein themetal oxide comprises zirconium dioxide.
 11. The coating agent of claim10, wherein the agent comprises more than about 70% by weight ofzirconium dioxide, based on a total weight of the coating agent.
 12. Thecoating agent of claim 11, wherein from about 80% to about 98% by weightof zirconium oxide are present.
 13. The coating agent of claim 12,wherein from about 90% to about 97% by weight of zirconium oxide arepresent.
 14. The coating agent of claim 8, wherein the agent comprisesat least one of methylcellulose and microsilica as additives.
 15. Thecoating agent of claim 14, wherein the agent comprises at least one of(a) from about 0.1% to about 1% by weight of methylcellulose and (b)from about 1% to about 10% by weight of microsilica, based on a totalweight of the agent.
 16. The coating agent of claim 15, wherein theagent comprises at least one of (a) from about 0.2% to about 0.7% byweight of methylcellulose and (b) from about 2% to about 8% by weight ofmicrosilica.
 17. The coating agent of claim 8, wherein the agentcomprises from about 15% to about 65% by weight of sodium silicate glassas liquid component, based on a total weight of oxide phase and the oneor more additives.
 18. The coating agent of claim 17, wherein from about20% to about 60% by weight of sodium silicate glass are present.
 19. Thecoating agent of claim 8, wherein a grain size of the oxide phase isfrom about 1 μm to about 50 μm.
 20. The coating agent of claim 19,wherein a mean grain size of the oxide phase is about 12.5 μm.